J.C. Penney CEO Ron Johnson admitted to making “some big mistakes” last year and took responsibility for the company’s 25 percent sales decline and losses of almost $1 billion.

J.C. Penney Co. chief executive Ron Johnson is taking the blame for some rotten results last year, but he isn’t about to rip up his playbook.

On Wednesday, the Plano-based retailer posted losses of almost $1 billion in 2012. Sales plunged just below $13 billion, $4.3 billion less than the previous year. That’s the equivalent of a company the size of Dallas-based Neiman Marcus disappearing in one year.

Despite promotions such as free family events and discounting the whole store on Black Friday, Penney’s fourth quarter was still a huge disappointment.

The holiday period ended the first full year of Johnson’s plan to transform J.C. Penney stores into a collection of shops by 2015.

Same-store sales declined 32 percent in the quarter, having worsened as the year progressed. Traffic was down 17 percent in the fourth quarter, worse than the full-year decline of 13 percent.

“I told you this would be a multiyear effort, and it will be,” Johnson said. “I told you transformations are unpredictable and can be bumpy, and this one has been.

“But our resolve has never been higher, and we greatly look forward to year two of our transformation.”

Markdowns affected Penney’s chances to generate profits. Johnson said merchandise had to go to make room for new brands. The Canadian apparel retailer Joe Fresh is opening shops in women’s apparel on March 15. The brand has been available on jcp.com since Sunday, and Johnson said that, so far, it is eclipsing sales of other brands online.

In May, home departments debut with shops from Martha Stewart, Jonathan Adler and Sir Terence Conran.

About 11 million square feet of Penney’s stores will be under construction this spring. During the year, the chain plans to add 60 Sephora shops, bringing the total to 446.

During a conference call with analysts, Johnson reviewed the company’s progress, such as building a new management team, adding shops in 700 of its largest stores, improving the look of all 1,100 stores, cutting costs by $800 million and ditching outdated systems.

Then he admitted “some big mistakes.”

Johnson said he takes personal responsibility for the dreadful results and said he has “learned a lot.”

“I had conviction to deliver the true price of an item on the tag,” he said. But we learned that the shopper “prefers a sale.”

“We have brought back sales,” he said but added that Penney isn’t going to go back to its old cadence of 590 promotions in one year.

That may translate into more of what Penney has done in the last month. Instead of weekly circulars with merchandise at everyday low prices, Johnson said items will be on sale. But most of what’s on sale will be Penney’s own brands.

Pressed by analysts to say when Penney will return to growth, Johnson said he wasn’t going to offer guidance. Previously, he had said growth would happen sometime later this year, after the home department is in place. Home represented more than 20 percent of sales at one time but has dropped to 10 percent.

Johnson did say that he was pleased with February results, and that promotions drove traffic.

Penney said it had $930 million in cash at the end of its fiscal year on Feb. 2. Johnson had promised that Penney would have $1 billion in cash at the end of the year. Analysts didn’t hammer Johnson about liquidity issues.

Online sales fell 34.4 percent from the same quarter last year to $315 million.

Fourth-quarter results were hurt by one-time charges including $148 million to coverlump-sum pension payments to people who were fired or took early retirement.

Penney reported a quarterly net loss of $552 million, or $2.51 a share, compared with a loss of $87 million, or 41 cents, in the same period a year ago.

Total fourth-quarter sales fell 28.4 percent to $3.88 billion from $5.43 billion a year earlier. Analysts surveyed by Thomson Reuters had forecast a loss of 18 cents a share and a sales decline of 25 percent to $4.08 billion.

“I don’t know how they’re going to provide convincing evidence that the program could be carried out, but I’ll be listening carefully,” Gilford Securities analyst Bernie Sosnick told Bloomberg News before the results were out. “It’s the worst performance that I’ve ever encountered in decades of covering retail — there’s nothing really to compare it against.”

For the year, Penney reported a net loss of $985 million, or $4.49 a share. Total sales were down 24.8 percent to $12.96 billion.

It was the sixth consecutive quarter of losses.

The results were released after the market closed. Penney’s shares were off almost 15 percent in after-hours trading.